This application incorporates by reference Taiwanese application Serial No. 89121139, filed on Oct. 9, 2000.
1. Field of the Invention
The invention relates in general to a method and an apparatus for forming images, and more particularly to a method for forming images with inkjet printing techniques and an apparatus therefor.
2. Description of the Related Art
Over the years, electronic related industries progress as the technology advances. For various electronic products, such as computer systems, computer peripherals, appliances and office machines, their functions and appearances are improved greatly as well. For example, in the 1980s, impact-type dot matrix printers and monochrome laser printers were pre-dominant. Later in the 1990s, monochrome inkjet printers and color inkjet printers became popular for common uses while color laser printers were available for professional uses. For common end users who do not print documents frequently, they would probably select color inkjet printers after considering the printing quality and price. People with sufficient budgets would probably purchase a monochrome laser printer. Since the price and quality are critical to the users"" choices, printer vendors aggressively develop their products so that the products have lower cost and better quality so as to increase popularity and profits of their products. Therefore, developers are focusing on how to improve the performance of products under limited cost.
Most inkjet printers now use thermal inkjet print head or piezo-electrical inkjet print head to spray ink droplets onto a sheet of medium, such as paper, for printing. The thermal inkjet print head includes ink, heating devices, and nozzles. The heating devices are to heat the ink to create bubbles until the bubbles expand enough to burst so that ink droplets are fired onto the sheet of paper through the nozzles and form dots or pixels on the sheet of paper. Varying the sizes and locations of the ink droplets can form different texts and graphics on a sheet of paper.
The quality of printing is closely related to the resolution provided by the printers. Currently, entry-level color printers provide a maximum resolution of 720 by 720 dot per inch (dpi) or 1440 by 720 dpi. Higher resolution requires finer size of the droplets. The size of the droplets is related to the cohesion of the ink. For instance, for droplets having identical amount of ink, ink with greater cohesion may have a smaller range of spread when they fall onto the paper, resulting in clearer and sharper printing. In the process of printing with the thermal inkjet technology, the heating elements of a print head are activated to heat up the ink in the print head for the creation of bubbles so that ink droplets are ejected from the nozzles onto a sheet of paper. As the temperature of the ink rises, the viscosity of the ink becomes lower. If the temperature of the ink is higher than a predetermined level, the viscosity of the ink could be abnormally low and ink droplets to be ejected would form larger dots onto the sheet of paper, resulting in a degraded quality of printing. Thus, the temperature control of the ink is a key to the improvement of the printing quality.
Referring to FIG. 1, it shows a block diagram of the structure of a convention inkjet printer. The inkjet printer includes a central processing unit (CPU) 110, a printing controller 120, a print head driver 130, and a print head 140. During printing, data representative of images to be printed are fed into the inkjet printer. After processing of the data, the CPU 120 feeds image data 115 into the printing controller 120. The image data 115 includes information of locations, colors, and density of pixels corresponding to the images to be printed. In response to the image data 115, the printing controller 120 controls the print head driver 130 and the print head driver 130 causes the print head 140 to print the images. Referring to FIG. 2, it gives an illustration of a portion of nozzles arranged on the print head 140. For the sake of simplicity, the nozzles of the print head 140 are represented as an array of nozzles 140. The print head 140 includes a plurality of nozzles and heating elements, and each of the heating elements is disposed in proximity to an associated nozzle to heat ink close to the nozzle for the ejection of ink droplets.
In course of printing, a nozzle may eject ink droplets consecutively. The heat generated by the heating element associated with the nozzle may accumulate because consecutive triggering signals are applied to the heating element while there is no enough time for the heat produced to release completely. Besides, the ink temperature near the nozzle may also be greater than that near the other nozzles. If the heat accumulation is not well compensated, the ink temperatures near different nozzles will be different from each other. The ink near different nozzles may have different viscosity. The ink droplets ejected from different nozzles would be of different sizes, resulting in a degraded printing quality. Thus, temperature compensation is necessary for improving the printing quality of thermal inkjet printing.
Conventional, there are two techniques for temperature compensation for use in inkjet printing apparatuses. In the first approach, temperature compensation is based on the temperature of the nozzles measured by a thermal resistor arranged near the nozzles. In addition, the temperature of the nozzles is determined by the variation of the resistance of the thermal resistor. However, the temperature obtained in this way is an average temperature of a part or all of the nozzles whereas the temperatures of specific nozzles are unobtainable. In other words, if abnormal temperature increase is observed, it is still not possible to identify the specific nozzles that cause the temperature rise in such conventional approach. Therefore the temperature compensation actions taken may not be appropriate.
In the second approach, temperature compensation is based on predictions about heat accumulation while the predictions are made by analyzing pixels of the image desired to be printed. If the formation of the images on a sheet of printing medium requires the ejection of a large number of ink droplets corresponding to the pixels of the images, a high degree of heat accumulation is expected. Conversely, if the formation of the images on the sheet of printing medium requires the ejection of a small number of ink droplets corresponding to the pixels of the images, a low degree of heat accumulation is expected. During printing, in order to achieve temperature compensation, evaluation of energy applied to each of the nozzles is made in accordance with the predications about heat accumulation. However, during consecutive ejection of ink droplets, heat release of the nozzles is incomplete so the heat accumulation is still happening in each nozzle. Thus, the second approach is unable to effectively resolve the problem on heat accumulation in the nozzles.
From the discussions above, it can be understood that the conventional temperature compensation approaches have two major disadvantages as follows.
1. Only the average temperature of a part or all of the nozzles of a print head is obtainable in the first approach, so the temperature compensation may be inadequate to reduce the effects of abnormal temperature variations in individual nozzles.
2. During consecutive ejection of ink droplets, though temperature compensation of the second approach is performing, there is still heat energy remaining in the nozzles, and the effects of heat accumulation is thus not effectively resolved.
It is therefore an object of the invention to provide a method and an apparatus for forming an image with inkjet printing techniques so that the degrading effects of heat accumulation on printing quality is reduced and the printing quality can thus be improved. According to the invention, data representative of the image is separating into m pieces of image data representing m sub-images and the m sub-images are printed successively according to the m pieces of image data. Besides, the data representative of the image can be adjusted for the reduction in heat accumulation during printing the sub-images.
The invention achieves the above-identified object by providing an image forming method for use in an inkjet device for forming an image on a printing medium. The image forming method includes the following steps. Firstly, provide data representative of the image. Then, m data masks for masking the data representative of the image is provided so as to obtain m pieces of image data representing m sub-images, wherein m is an integer greater than one. The m sub-images is then printed according to the m pieces of image data representing the m sub-images so that the m sub-images are superimposed on the printing medium, whereby the image is formed on the printing medium.
Besides, the degree of heat accumulation during printing is predicted by determining a heat weighting for the image based on the locations of the pixels to be printed for the image. Further, for one of the m pieces of image data which has some pixels to be printed may cause serious heat accumulation during printing, the densities of these pixels to be printed can be adjusted so as to reduce the effect of the heat accumulation on the printing quality. The effect of the heat accumulation on the printing quality can also be reduced by reducing the densities of the data representing the image according to the degree of heat accumulation predicted before the obtaining of the m pieces of image data.
The invention achieves the above-identified object by providing an apparatus for controlling inkjet printing. The apparatus includes a memory, a heat accumulation calculation device, and an image separating device. The memory is used to store a heat weighting look-up table. The heat accumulation calculation device is coupled to the memory and is used for receiving data representative of an image and outputting a heat weighting for the image according to the heat weighting look-up table. The image separating device is coupled to the heat accumulation calculation device and is used for receiving the data representative of the image and outputting m pieces of image data representing m sub-images according to the heat weighting for the image.
Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments of the invention with reference to the accompanying drawings.